It is well known to produce elastomeric compounds such as those used in seals, gaskets, tyres, cables and other articles made from rubber using ingredients like fillers, plastizisers, antioxidants, curatives and others. All these compound ingredients are used to receive certain properties of the final article or are necessary during the manufacturing. But some of these substances do influence each other leading to detrimental effects on physical properties or on processing behaviour. Functional fillers, for instance some carbon blacks and precipitated silicas, are used to improve hardness, tensile strength, tear resistance and other desired properties, but they also, especially at high filler loadings, increase compound viscosity leading to poor processability and scorch safety. This has to be balanced out by the incorporation of plastizisers and/or process aids. Plastizisers and process aids have, however, a negative influence on physical properties, including fire performance and they can “bloom out”. So far no material is known that could overcome the processing difficulties of elastomeric resin compositions with high filler loading, and still maintaining the desired physical properties.
It is very difficult to give a precise definition of “high filler loading” as it depends very much on the polymer used and on the application of the final compound. However, generally if the filler content of a highly filled elastomeric compounds is increased, the viscosity will increase to a level where the processability of the compounds will be strongly reduced. The amount of fillers in highly filled resins can thus, depending on the polymer, vary from about 15 to about 500% by weight of resin.
It is known to use microsilica as semi-reinforcing filler in elastomers replacing for instance MT (medium thermal)—black or calcium silicate fillers. In these instances microsilica has always been used as a replacement to obtain a less costly elastomer having the same strength. Thus the total filler loading has never been increased when using microsilica as a semi-reinforcing filler.